You can even have your party fight Cthulhu if you want (though they'll lose)! I remember reading the playtest notes somewhere about the WotC developers running a combat with four level-20 PCs facing off against Cthulhu. When one of the PCs died, they were allowed to bring another one in on the next round. If I remember correctly, they said that they went through about 12 PCs before they finally managed to get Cthulhu to fail his save against an Imprisonment spell. This ended the combat, but was a "temporary" solution at best.